Cobalt oxide underglazes

[limiacrafts]

Hello!

I am making tests with cobalt oxide underglazes and I am still struggling with making my own formula. I am making a very delicate linework and I want to ask for any tip to prevent running lines.

For making my own underglaze I am mixing cobalt oxide with CMC and 10% frit called PR500 here in Spain (it is a low temperature non leaded glaze). I paint my pieces on leather hard clay (greenware) and fire them in low temperature (1050 *C).

I was reading about making cobalt oxide underglaze formula by adding Gerstley Borate to the mixture, but I am afraid this material is no longer available on the market. I have also read that adding Kaolin to the cobalt oxide powder will help to avoid running lines.

I will really appreciate if you have any tips on how to make a trustworthy formula to paint thin lines with cobalt oxide underglazes.

Thank you so much!

[Min]

Hi and welcome to the forum.

I'm not familiar with a frit named PR500 so I did a quick search for it. The only thing that came up was a fritted glaze called PR500, link to it here.  Just confirming this is the same material.

If is is then what you are doing is by adding cobalt oxide you are adding what is a very strong flux in a high boron glazes (which PR500 will be as it's a nonleaded lowfire glaze). Fluxes melt/dissolve the oxides within a glaze.  If the glaze itself is a fluid one (runny) then adding the cobalt will cause the brushwork to move/blur.

There are a few ways around this, first off just use cobalt oxide + water for the brushwork, bisque fire the pots then glaze fire. Problem with this is the brushwork is susceptible to smudging before being fired. Another option would be to mix cobalt oxide + kaolin/china clay + some frit. (not your fritted glaze but a frit like Ferro 3124 or something similar that is available in Spain) Third option would be a 1:1:1 mix of cobalt oxide + Ferro frit 3124 + bentonite. The bentonite will drastically thicken the mixture and also help harden it when it dries.

Again, make sure you use as stiff a covering glaze as possible, you don't want the glaze to run and pull the brushwork with it during the glaze firing.

 

[Mark C.]

I have a few tips

You will get less movement of lines working on bisque ware than greenware-meaning less drift of blue line

Also the Gerstly borate now has a substitute being made but I would avoid all that

I personally when doing line drawings with cobalt just use water with coblat  mixed in a jar(no glaze or Frits) and let it dry and then dip a really thin clear over it. You can also spray that clear   glaze thinly.

I do this on bisque ware

you can get crisp lines this way-the key is not ever touch the drawing section when its done

here is one on porcelain fired to cone 10 from long ago in the 80s

Edited August 30 by Mark C.

[limiacrafts]

That is really helpful! Thank you so much for your tips.

I was looking for a frit similar to Ferro3124 here in Spain and I found a couple ones that have sort of similar ingredients. I will make some tests with frit 5005 prodesco and with frit C-551 prodesco.

I think the biggest issue with my old tests was the glaze (pr500) I was using, because it is really runny and it moves the line away.

I have made in the past different tests with cobalt oxide and none of them was really convincing..... I painted just with watter and cobalt oxide over bisque pieces, I painted on a bisque piece over raw glaze, I have tried with a couple of different glazes....

I attach here some test I made painting over a matt raw glaze, which was kind of ok, but I don`t like the process of painting over raw glaze.... the brush does not run very well (even using an special majolica brush).

So, for the moment, I am going to make some new test using different glazes. I will use this new frits I mentioned above for making my underglaze mixes and also for the final glaze layer. I will also make some tests adding kaolin to the underglaze mix. And lets see how it goes! I am really exited about it!

I want to thank you again for your help! It is much appreciated!

And I will post here some photos when the new tests are done.

Best regards

Elara Elvira

 

 

Edited September 1 by limiacrafts

[Min]

21 minutes ago, limiacrafts said:

I will make some tests with frit 5005 prodesco and with frit C-551 prodesco.

There is lead in both of those frits, can you find a lead free frit instead?

edit: If you want to do majolica style brushwork on top of a glaze then adding some brushing medium (either bought or made) will help the flow, as will adding glycerine to the cobalt + water. Spraying the pot with hairspray or a spray laundry starch also helps put a "crust" on the raw glaze so this too helps with brushwork.

Edited September 1 by Min

[Mark C.]

Here is another one from same time frame-the clear glaze overf should be very thinly  applied-this pot it a lidded form with another glaze on the edges and sides.

Edited September 1 by Mark C.

[Mark C.]

I need to mention I made these pots but did NOT do the drawings. Yes they are my sea themes for sure but a friend does that drawing. She is a great drawer. She is going to do some cat bowls that I threw today for my upcoming art show in a few weeks.I'll try to get a photo. They are simple compared to the pots I have shown so far. I band them and she draws a simple cat face in bottom.

Edited September 2 by Mark C.

[Babs]

You can get fine tipped  applicators, practise on newspaper to get the tech. of flowing onto surface, no or v minimal touch is the go. Is your clay lumpy or the glaze?

Be difficult to get a goid application in that rough surface imo.

Electrical tape or I have seen rubber bands used to get a neat line.

[limiacrafts]

14 hours ago, Min said:

There is lead in both of those frits, can you find a lead free frit instead?

edit: If you want to do majolica style brushwork on top of a glaze then adding some brushing medium (either bought or made) will help the flow, as will adding glycerine to the cobalt + water. Spraying the pot with hairspray or a spray laundry starch also helps put a "crust" on the raw glaze so this too helps with brushwork.

Hello Min! is the lead free glaze gonna help with by making a stiffer covering? Because I am not making utilitarian pieces, so, in that sense, I am not really worried about having lead on them.

Thank you so much for the tips on majolica techniques! I will be using them when making my next tiles!

[limiacrafts]

5 hours ago, Babs said:

You can get fine tipped  applicators, practise on newspaper to get the tech. of flowing onto surface, no or v minimal touch is the go. Is your clay lumpy or the glaze?

Be difficult to get a goid application in that rough surface imo.

Electrical tape or I have seen rubber bands used to get a neat line.

Hello! Nor the glaze or the clay are lumpy.

I know it is not common to paint with cobalt oxide over raw clay, but I think I am getting a formula to do it, because I really like how the brush works over it, so I will keep on with my researches and let you know when I have some new results!

[Babs]

10 minutes ago, limiacrafts said:

Hello! Nor the glaze or the clay are lumpy.

I know it is not common to paint with cobalt oxide over raw clay, but I think I am getting a formula to do it, because I really like how the brush works over it, so I will keep on with my researches and let you know when I have some new results!

The 3rd cylinder from left looks lumpy to me....

Cobalt on unfired clay common practice, like majolica also common practice.

[limiacrafts]

4 minutes ago, Babs said:

The 3rd cylinder from left looks lumpy to me....

Cobalt on unfired clay common practice, like majolica also common practice.

The lumpy effect is because of the "chamotte" present on the clay. This sculpture was fired one time and then, once bisqued, it was painted with cobalt oxide over a raw glaze.

[Babs]

1 hour ago, limiacrafts said:

The lumpy effect is because of the "chamotte" present on the clay. This sculpture was fired one time and then, once bisqued, it was painted with cobalt oxide over a raw glaze.

Great info on the link I posted.

If chamotte aka grog is in the clay the surface will be" lumpy" not smooth depending on the grade of the chamotte?? Depending on how fine detailed your work is will  dictate how this surface will affect your success of application.

[Min]

If you are interested in reading up some more on majolica an excellent source of info is from Linda Arbuckle, link to one of her handouts here. More info on her website https://www.lindaarbuckle.com/  (the menu is at the upper left side on the painted leaves)

[Mark C.]

Here is some of the recent (a day ago ) cobalt  brush work. I did made the  pots and the cobalt banding and glazing another drew the cat faces. They will glaze fire in am. This brush gives a fine line as well. its just a cheap bamboo brush I have used for this type of work for many decades-maybe 4  ok 5 of them.

 

 

Edited September 7 by Mark C.

[limiacrafts]

On 9/2/2024 at 4:39 PM, Min said:

If you are interested in reading up some more on majolica an excellent source of info is from Linda Arbuckle, link to one of her handouts here. More info on her website https://www.lindaarbuckle.com/  (the menu is at the upper left side on the painted leaves)

It has really interesting formulas here, thank you so much!

[limiacrafts]

On 9/2/2024 at 12:06 PM, Babs said:

Great info on the link I posted.

If chamotte aka grog is in the clay the surface will be" lumpy" not smooth depending on the grade of the chamotte?? Depending on how fine detailed your work is will  dictate how this surface will affect your success of application.

Yes, I had already read that post before posting my question. But this post you send is mainly talking about cobalt carbonate, not cobalt oxide, which is what I am investigating. Thanks anyway.

 

[Mark C.]

For line drawing either cobalt oxide or carbonate will work-the only difference is strength of color-all other factors are the same.

[Mark C.]

Ok the  cobalt line drawings came out of  2 cone 10 fires today. I have offered cat bowls at my upcoming fair for about 35 +years now with this year a more cat conciseness  due to recent events that have nothing to do with my cat bowls. This upcoming show in two weeks will be my 50 year at show and its the shows 50th yearas well. also made some fish for the show as well.I even have a few cat sponge holders which have rubber no skid bottoms .

 

 

Edited September 10 by Mark C.